1. Field of the Invention
The invention relates to an X-ray generator for operating an X-ray tube with parts of the tube connected to ground and a high-voltage transformer arrangement comprising distinct secondary windings for producing the positive and negative high voltages, respectively, for the anode and the cathode, respectively, of the X-ray tube. Such an X-ray generator is known from EP-B 74141 which corresponds to U.S. Pat. No. 4,439,869.
2. Description of the Prior Art
In X-ray tubes having a part of the tube connected to ground, for example, a metal envelope which optionally may be connected to a metal part present between the anode and the cathode, the current produced at the cathode does not flow completely to the anode; a part of the current flows to ground via the part of the tube in question. As a result of this, the high-voltage source on the cathode side is loaded more strongly than the high-voltage source on the anode side, which in high-ohmic symmetrically arranged high-voltage sources leads to an asymmetry between the high voltage on the anode and the cathode, respectively, (i.e. the high voltage between anode and ground exceeds the high voltage between cathode and ground).
This asymmetry involves negative effects which depend on the value of the voltage between anode and cathode:
a) at high tube voltages the voltage between anode and ground reaches a value of more than half of the maximally permissible tube voltage before the voltage between anode and cathode reaches its maximally permissible value. In order to avoid a high-voltage overload of the X-ray tube, the X-ray tube may in such a case not be operated with the full voltage for which it is designed. PA1 b) at low tube voltages the cathode voltage may become so low that the current emitted at the cathode is limited by space charge effects. In order to reach a given tube current the filament current for the cathode must in this case be made unnecessarily large, which may lead to a reduction of the life of the tube.
In the known X-ray generator the voltage asymmetry and the negative effects produced thereby are removed in that a high-voltage transformer is provided having a primary winding and three secondary windings each with a rectifier. The three rectifier outputs are connected together via a switching device in such a manner that the high voltage of the anode side is optionally produced by one or two rectifiers and the high voltage of the cathode side conversely by two and one rectifier, respectively. The cost for this solution (additionally one secondary winding, one high-voltage rectifier and one high-voltage switching device) is comparatively high.